1. Field of the Invention
The present invention relates to a technique of controlling the optical power of an optical beam, more specifically, to a scanning apparatus, image forming apparatus and optical power control method.
2. Description of the Related Art
Generally, it is desired that an optical beam scanning apparatus or image forming apparatus accurately controls the optical power of a laser beam or the like.
An APC (Auto Power Control) circuit described in Japanese Patent Laid-Open No. 8-330661 causes a light-receiving element to monitor a laser beam (front side light) split by a half mirror and controls the optical power based on the result of monitoring. This APC scheme will be called a front side light APC scheme.
In the front side light APC scheme, however, it is necessary to place a half mirror in an optical system to split a beam into transmitted light and reflected light. Hence, the efficiency of optical power use (optical power used for exposure/total optical power) becomes low.
Japanese Patent Laid-Open No. 6-164070 proposes another front side light APC scheme without a half mirror in an optical system. According to this APC scheme, a light-receiving element is arranged to receive a part (leakage light) of the spot of a beam output from a laser. The leakage light is cut off by a beam shaping slit and is not used for exposure. The APC circuit controls the optical power based on the optical power of the leakage light obtained by the light-receiving element. This APC scheme will be called a leakage light APC scheme. The leakage light APC scheme required no half mirror. Hence, the efficiency of optical power use can be improved as compared to the front side light APC scheme using a half mirror.
However, in the conventional leakage light APC scheme, the optical power (exposed optical power) at the central part of the spot and that (leakage optical power) at the peripheral part have a nonlinear relationship. That is, when the exposed optical power is controlled by using the leakage optical power, a control error may occur. The control error is undesirable because it, for example, degrades the quality of a formed image.